feat: add prestige system and skill upgrades with comprehensive documentation

docs/task5/subtask_17_context.md

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+# Task 17: Fix Skill Requirement Display Bug (undefined Lv.[object Object])
+
+## Summary
+
+This bug causes skills to display "Requires: [Skill Name] Lv.[object Object]" instead of the proper level requirement. The root cause is that `cost` objects are incorrectly placed INSIDE the `req` (requirements) object in `skills.ts`, causing the rendering code to iterate over the cost object as if it were a skill requirement.
+
+## Root Cause
+
+In `/home/user/repos/Mana-Loop/src/lib/game/constants/skills.ts`, several skill entries have the `cost` property incorrectly nested inside the `req` object.
+
+### Correct Format (cost OUTSIDE req):
+```typescript
+researchFireSpells: { 
+  name: "Fire Spell Research", 
+  desc: "...", 
+  cat: "effectResearch", 
+  max: 1, 
+  base: 300, 
+  studyTime: 6, 
+  req: { enchanting: 2 }, 
+  cost: { type: 'element', element: 'fire', amount: 100 }, 
+  attunementReq: { enchanter: 1 } 
+}
+```
+
+### Malformed Format (cost INSIDE req - BUG):
+```typescript
+researchLifeDeathSpells: { 
+  name: "Death Research", 
+  desc: "...", 
+  cat: "effectResearch", 
+  max: 1, 
+  base: 400, 
+  studyTime: 8, 
+  req: { enchanting: 3 , cost: { type: 'element', element: 'death', amount: 100 }},  // BUG: cost inside req
+  attunementReq: { enchanter: 2 } 
+}
+```
+
+## Malformed Skill Entries
+
+The following skills have `cost` incorrectly placed inside `req` (lines in skills.ts):
+
+1. **researchLifeDeathSpells** (line 51)
+2. **researchAdvancedFire** (line 54)
+3. **researchAdvancedWater** (line 55)
+4. **researchAdvancedAir** (line 56)
+5. **researchAdvancedEarth** (line 57)
+6. **researchAdvancedLight** (line 58)
+7. **researchAdvancedDark** (line 59)
+8. **researchMasterFire** (line 62)
+9. **researchMasterWater** (line 63)
+10. **researchMasterEarth** (line 64)
+11. **researchMetalSpells** (line 86)
+12. **researchSandSpells** (line 87)
+13. **researchLightningSpells** (line 88)
+14. **researchAdvancedMetal** (line 91)
+15. **researchAdvancedSand** (line 92)
+16. **researchAdvancedLightning** (line 93)
+17. **researchMasterMetal** (line 96)
+18. **researchMasterSand** (line 97)
+19. **researchMasterLightning** (line 98)
+
+Total: **19 malformed skill entries**
+
+## How Skill Requirements are Rendered
+
+In `/home/user/repos/Mana-Loop/src/components/game/tabs/SkillsTab.tsx` (lines 233-235):
+
+```tsx
+{!prereqMet && def.req && (
+  <div className="text-xs text-red-400 mt-1">
+    Requires: {Object.entries(def.req).map(([r, rl]) => `${SKILLS_DEF[r]?.name} Lv.${rl}`).join(', ')}
+  </div>
+)}
+```
+
+The code expects `def.req` to be `Record<string, number>` (skill ID -> required level). When `cost` is inside `req`, the iteration produces entries like:
+- `[ "enchanting", 3 ]` → "Enchanting Lv.3" ✓
+- `[ "cost", { type: 'element', ... } ]` → "undefined Lv.[object Object]" ✗
+
+## The Fix
+
+For each malformed entry in `skills.ts`, move the `cost` property OUT of the `req` object:
+
+**Before:**
+```typescript
+req: { enchanting: 3 , cost: { type: 'element', element: 'death', amount: 100 }}
+```
+
+**After:**
+```typescript
+req: { enchanting: 3 }, cost: { type: 'element', element: 'death', amount: 100 }
+```
+
+## Files to Modify
+
+1. `/home/user/repos/Mana-Loop/src/lib/game/constants/skills.ts` - Fix 19 malformed skill entries
+
+## No Changes Needed In
+
+- `/home/user/repos/Mana-Loop/src/components/game/tabs/SkillsTab.tsx` - Rendering code is correct
+- `/home/user/repos/Mana-Loop/src/lib/game/formatting.ts` - Not related to this bug
+- `/home/user/repos/Mana-Loop/src/lib/game/types/skills.ts` - Type definitions are correct
+
+## Steps to Fix
+
+1. Edit `/home/user/repos/Mana-Loop/src/lib/game/constants/skills.ts`
+2. For each of the 19 malformed entries, move `cost` from inside `req` to be a separate property
+3. Verify the fix by checking that `Object.entries(def.req)` only returns skill ID/level pairs
+4. Run typecheck and lint to confirm no errors